Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process. This is a three-step process in which the first step involves alkylation of benzene with propylene to produce cumene, followed by oxidation of the cumene to the corresponding hydroperoxide and then cleavage of the hydroperoxide to produce equimolar amounts of phenol and acetone. However, the world demand for phenol is growing more rapidly than that for acetone. In addition, due to a developing shortage, the cost of propylene is likely to increase. Thus, a process that uses higher alkenes instead of propylene as feed and co-produces higher ketones, rather than acetone, may be an attractive alternative route to the production of phenols.
One such process involves the hydroalkylation of benzene to produce cyclohexylbenzene, followed by the oxidation of the cyclohexylbenzene (analogous to cumene oxidation) to cyclohexylbenzene hydroperoxide, which is then cleaved to produce phenol and cyclohexanone in substantially equimolar amounts. Such a process is described in, for example, U.S. Pat. No. 6,037,513.
However, one problem in producing phenol by way of the cleavage of cyclohexylbenzene hydroperoxide is that the cyclohexanone and phenol produce an azeotropic mixture composed of 28 wt % cyclohexanone and 72 wt % phenol. Thus any attempt to separate the cleavage effluent by simple distillation results in this azeotropic mixture. To obviate this problem it has been proposed to integrate the cyclohexylbenzene oxidation and cleavage process with a dehydrogenation step whereby at least part of the cyclohexanone is converted to additional phenol (see International Patent Publication No. WO2010/024975). Such a dehydrogenation step is generally achieved by contacting the cyclohexanone with a supported noble metal catalyst at a temperature of about 250° C. to about 500° C.
For example, U.S. Pat. No. 3,534,110 discloses a process for the catalytic dehydrogenation of cyclohexanone and/or cyclohexanol to phenol over a catalyst comprising platinum and preferably iridium on a silica support. The catalyst also contains 0.5 wt % to 3 wt % of an alkali or alkaline earth metal compound, which, according to column 3, lines 43 to 49, should be incorporated after addition of the platinum since otherwise the resulting catalyst composition has inferior activity, selectivity, and life.
In addition, U.S. Pat. No. 3,580,970 discloses a process for the dehydrogenation of cycloaliphatic alcohols and ketones to the corresponding hydroxyaromatic alcohols in the presence of a catalyst comprising a Group VIII metal, particularly nickel, and tin in a molar amount of about 1.7 to about 15 moles of Group VIII metal per mole of tin. The catalyst may further comprise a silica support and an alkali metal stabilizing agent. In the Examples, the catalyst contains between 2.22 wt % and 14.2 wt % tin.
U.S. Pat. No. 4,933,507 discloses that phenol can be produced by dehydrogenating cyclohexenone through a vapor-phase reaction in the presence hydrogen using a solid-phase catalyst having platinum and alkali metal carried on a support, such as silica, silica-alumina or alumina. The catalyst is prepared by first treating the support with an aqueous solution of chloroplatinic acid, etc. to have platinum chloride carried on the support, and then treating the support to have an alkali metal compound such as K2CO3 supported thereon, and finally reducing the so treated support. The content of alkali metal in the catalyst is preferably in the range of 0.5 wt %-2.0 wt % in terms of Na2O based on the weight of the support and in the range of 0.2 wt %-3.0 wt % in terms of K2CO3 based on the weight of the platinum.
U.S. Pat. No. 7,285,685 discloses a process for the dehydrogenation of a saturated carbonyl compound, such as cyclohexanone, in the gas phase over a heterogeneous dehydrogenation catalyst comprising platinum and/or palladium and tin on an oxidic support, such as zirconium dioxide and/or silicon dioxide (SiO2). In general, the dehydrogenation catalyst contains from 0.01 wt % to 2 wt %, preferably from 0.1 wt % to 1 wt %, particularly preferably from 0.2 wt % to 0.6 wt %, of palladium and/or platinum and from 0.01 wt % to 10 wt %, preferably from 0.2 wt % to 2 wt %, particularly preferably from 0.4 wt % to 1 wt %, based on the total weight of the dehydrogenation catalyst. In addition, the dehydrogenation catalyst can further comprise one or more elements of Groups I and/or II, preferably potassium and/or cesium, which are added to the catalyst as aqueous solutions of compounds which can be converted into the corresponding oxides by calcination. In the only catalyst preparation Example, an aqueous solution containing CsNO3 and KNO3 is added to a silica/titania support after the support has been impregnated with a solution of SnCl2.2H2O and H2PtCl6.6H2O in ethanol, then dried at 100° C. for 15 hours and calcined at 560° C. for 3 hours.
One problem with existing cyclohexanone dehydrogenation catalysts is that they also tend to catalyze the competing hydrogenolysis of cyclohexanone to produce pentane, pentene and carbon monoxide. This not only leads to loss of valuable cyclohexanone, but also the pentene can react with benzene normally present in the cyclohexanone feed to produce pentylbenzene which is difficult to separate from the phenol product. In addition, any carbon monoxide produced passes into the co-produced hydrogen stream so that the hydrogen stream requires additional processing to reduce the CO to very low levels before the hydrogen can be recycled to, for example, the original benzene hydroalkylation step.
There is therefore a need for a cyclohexanone dehydrogenation catalyst having improved selectivity to the production of phenol and reduced selectivity to side reactions, such as hydrogenolysis to pentane, pentene and carbon monoxide.
According to the present invention, it has now been found that the addition of small amounts of tin to a supported, Group 6 to 10 metal-containing cyclohexanone dehydrogenation catalyst reduces the formation of pentylbenzene and carbon monoxide as well as improves the stability of the catalyst. Preferably, the tin is incorporated in the catalyst before the Group 6 to 10 metal.